Judging from video, the An-26 was shot down at altitude about 4400 m, not 6000 or more (perhaps claimed by crew). Also, its engine was hit, therefore IR homing. Buk has radar homing.

The reports I read said it was at 6500 metres. It was a Russian media outlet (TV Zvezda) that said it was a BUK that did it. They said the rebels had captured a BUK from Ukrainian forces. A claim we now know is nonsense. The launcher was supplied from over the border as the photographic and phone footage evidence shows.

Still, even if it wasn't a BUK (which I think it was) it proved that the rebels had a game-changer on their hands.

This, I think, shows up the culpability of Kiev for not closing the obviously dangerous airspace and the airlines that chose to continue to fly over the war zone despite the changed circumstances.